The invention relates to a steam generator having a first and a second combustion chamber with a respective number of burners for fossil fuel.
In a power plant having a steam generator, the energy content of a fuel is utilized for evaporating a flow medium in the steam generator. The steam generator has evaporator tubes which are heated, leading to evaporation of the flow medium conducted therein in order to evaporate the flow medium. Steam provided by the steam generator may in turn be used, for example, for a connected external process or for driving a steam turbine. If the steam drives a steam turbine, a generator or a driven machine is normally operated through a turbine shaft of the steam turbine. In the case of a generator, the current generated by the generator can be provided for feeding into an interconnected and/or separate network.
In that case, the steam generator may be constructed as a once-through steam generator. A once-through steam generator has been disclosed by a paper entitled xe2x80x9cVerdampferkonzepte fur Benson-Dampferzeugerxe2x80x9d [Evaporator Concepts For Benson Steam Generators] by J. Franke, W. Kxc3x6hler and E. Wittchow, published in VGB Kraftwerkstechnik 73 (1993), No. 4, pages 352-360. In a once-through steam generator, the heating of steam-generator tubes provided as evaporator tubes leads to evaporation of the flow medium in the steam-generator tubes in a single pass.
Once-through steam generators are normally constructed with a combustion chamber in a vertical type of structure. That means that the combustion chamber is constructed for a throughflow of a heating medium or heating gas in an approximately vertical direction.
In that case, a horizontal gas flue can be connected downstream of the combustion chamber on the heating-gas side. The heating-gas flow is deflected into an approximately horizontal flow direction at a transition from the combustion chamber into the horizontal gas flue. However, due to temperature-induced changes in length of the combustion chamber, the combustion chamber generally requires a framework on which the combustion chamber is suspended. That necessitates a considerable technical outlay during the manufacture and installation of the once-through steam generator, which becomes larger as the overall height of the once-through steam generator becomes larger.
Fossil-fuel-fired steam generators are normally constructed for a particular type and quality of fuel and for a certain output range. That means that the combustion chamber of the steam generator, in its main dimensions, that is length, width and height, is adapted to combustion properties and ash properties of the predetermined fuel and to the predetermined output range. Therefore, each steam generator, with its fuel and output range associated therewith, has an individual structure of the combustion chamber with regard to the main dimensions.
If the combustion chamber of a steam generator is to be reconstructed, for example for a new output range and/or a fuel of a different type or quality, recourse may be had to planning documents of already-existing steam generators. With the aid of the documents, the main dimensions of the combustion chamber are then normally adapted to the requirements of the steam generator to be reconstructed. However, despite that simple measure, the structure of a steam generator for newly predetermined boundary conditions, due to the complexity of the systems taken as a basis, still involves a comparatively high design cost. That applies in particular when the respective steam generator is to have an especially high overall efficiency.
It is accordingly an object of the invention to provide a fossil-fuel-fired steam generator, which overcomes the hereinafore-mentioned disadvantages of the heretofore-known devices of this general type, which has a concept for a combustion chamber that permits an especially simple structure for a certain type and quality of fuel and for a predetermined output range and which requires especially little outlay in terms of manufacture and installation.
With the foregoing and other objects in view there is provided, in accordance with the invention, a steam generator, comprising a modular combustion space including a first module having at least one first combustion chamber and a second module having at least one second combustion chamber. The at least one first and at least one second combustion chambers each have a respective number of burners for fossil fuel and define a substantially horizontal main flow direction for heating gas. A common horizontal gas flue is provided, into which the at least one first and at least one second combustion chambers lead. A vertical gas flue is disposed downstream of the common horizontal gas flue on the heating-gas side.
The invention is based on the idea that a concept for the combustion chamber of the steam generator should permit an especially simple construction for a certain type and quality of fuel and for a predetermined output range of the steam generator. This is the case if a modular type of construction of the combustion chamber is provided. In this case, modules of the same kind prove to be especially simple to handle and permit an especially high degree of flexibility with regard to a desired rating of the combustion chamber. In addition, it should be especially simple to increase or reduce the size of the combustion chamber through the use of the modules.
However, a combustion chamber constructed for a throughflow of the heating gas in an approximately vertical direction requires a framework which is technically very complicated to construct. That framework would also have to be appropriately adapted with considerable outlay if the steam generator was retrofitted. In contrast thereto, a framework which is to be constructed with comparatively little technical outlay can be accompanied by an especially low overall height of the steam generator. A combustion chamber given a horizontal type of construction and having a first and a second combustion chamber therefore offers an especially simple concept for a steam generator with a modular construction. In this case, the burners, in both the first and the second combustion chambers, are disposed at the level of the horizontal gas flue in the combustion-chamber wall. The heating gas therefore flows through the combustion chambers in an approximately horizontal main flow direction during operation of the steam generator.
The burners are advantageously disposed on an end wall of the first combustion chamber and on an end wall of the second combustion chamber, that is on that containing wall of the respective first and second combustion chambers which is opposite the outflow opening to the horizontal gas flue. A steam generator with such a construction can be adapted to the burn-out length of the fuel in an especially simple manner. The burn-out length of the fuel in this case refers to the heating-gas velocity in the horizontal direction at a certain average heating-gas temperature, multiplied by a burn-out time tA of the fuel. In this case, the maximum burn-out length for the respective steam generator is obtained during full load, that is the xe2x80x9cfull-load operationxe2x80x9d of the steam generator. The burn-out time tA is in turn the time which, for example, a pulverized-coal grain of average size requires in order to burn out completely at a certain average heating-gas temperature.
In order to keep material damage and undesirable contamination of the horizontal gas flue, for example due to a yield of molten ash at a high temperature, at an especially low level, a length L of the first and the second combustion chambers, which length is defined by a distance from the end wall to an inlet region of the horizontal gas flue, is advantageously at least equal to the burn-out length of the fuel during full-load operation of the steam generator. This horizontal length L of the first combustion chamber and of the second combustion chamber will generally be larger than the height of the respective first or second combustion chamber, measured from a funnel top edge up to the top of the combustion chamber.
In an advantageous refinement, the length L (specified in m) of the first and the second combustion chambers is selected for especially favorable utilization of the heat of combustion of the fossil fuel as a function of a BMCR value W (specified in kg/s) of the steam generator, of a number N of combustion chambers, of the burn-out time tA (specified in s) of the fuel and of an outlet temperature TBRK (specified in xc2x0C.) of the heating gas from the combustion chambers. The term BMCR stands for Boiler Maximum Continuous Rating. BMCR is the term normally used internationally for the maximum continuous output of a steam generator. That also corresponds to the design output, which is the output during full-load operation of the steam generator. In this case, at a given BMCR value W and a given number of combustion chambers N, the length L of the first and the second combustion chambers is approximately the larger value of two functions (1) and (2):
L(W, N, tA)=(C1+C2xc2x7W/N)xc2x7tAxe2x80x83xe2x80x83(1)
L(W, N, TBRK)=(C3xc2x7TBRK+C4)(W/N)+C5(TBRK)2+C6xc2x7TBRK+C7xe2x80x83xe2x80x83(2)
where:
C1=8 m/s and
C2=0.0057 m/kg and
C3=xe2x88x921.905xc2x710xe2x88x924(mxc2x7s)/(kg xc2x0C.) and
C4=0.286(sxc2x7m)/kg and
C5=3xc2x710xe2x88x924 m/(xc2x0C.)2 and
C6=xe2x88x920.842 m/xc2x0C. and
C7=603.41 m.
In this case, xe2x80x9capproximatelyxe2x80x9d is to be understood as an admissible deviation by +20%/xe2x88x9210% from the value defined by the respective function.
The end wall of the first combustion chamber and the end wall of the second combustion chamber as well as the side walls of the respective first and second combustion chambers of the horizontal gas flue and/or of the vertical gas flue are advantageously formed from vertically disposed evaporator tubes or steam-generator tubes which are welded to one another in a gas-tight manner. A flow medium can be admitted in a parallel manner to a respective number of evaporator or steam-generator tubes.
In order to provide especially good heat transfer of the heat of the first and the second combustion chambers to the flow medium conducted in the respective evaporator tubes, a number of evaporator tubes each advantageously has ribs on their inside forming a multi-start thread. In this case, a helix angle a between a plane perpendicular to the tube axis and the sides of the ribs disposed on the inside of the tube is advantageously less than 60xc2x0, preferably less than 55xc2x0.
This is because, in a heated evaporator tube constructed as an evaporator tube without inner ribbing, that is a xe2x80x9csmooth tubexe2x80x9d, the wetting of the tube wall which is required for especially good heat transfer, can no longer be maintained starting from a certain steam content. If there is a lack of wetting, there may be a tube wall which is dry in places. The transition to such a dry tube wall leads to a type of critical stage of the heat transfer with impaired heat-transfer behavior. Therefore, in general, the tube-wall temperatures at this location increase to an especially pronounced extent. However, in an inner-ribbed tube, this critical stage of the heat transfer, compared with a smooth tube, does not occur until there is a steam mass content  greater than 0.9, that is just before the end of the evaporation. This may be attributed to the swirl which the flow undergoes due to the spiral-shaped ribs. Due to their different centrifugal forces, the water portion is separated from the steam portion and forced onto the tube wall. As a result, the wetting of the tube wall is maintained up to high steam contents, so that there are already high flow velocities at the location of the heat-transfer critical stage. Despite the heat-transfer critical stage, this produces relatively good heat transfer and consequently low tube-wall temperatures.
A number of evaporator tubes of the combustion chamber advantageously have measures for reducing the throughflow of the flow medium. In this case, it proves to be especially favorable if the measures are provided as choke devices. Choke devices may, for example, be components built into the evaporator tubes. These built-in components reduce the inside diameter of the tube at a location in the interior of the respective evaporator tube. At the same time, measures for reducing the throughflow in a line system including a plurality of parallel lines also prove to be advantageous. The flow medium can be fed through the line system to the evaporator tubes of the combustion chamber. In this case, for example, choke fittings may be provided in one line or in a plurality of lines of the line system. With such measures for reducing the throughflow of the flow medium through the evaporator tubes, the rate of flow of the flow medium through individual evaporator tubes can be adapted to the respective heating in the combustion chamber. As a result, temperature differences of the flow medium at the outlet of the evaporator tubes can additionally be kept especially small in an especially reliable manner.
Respective adjacent evaporator or steam-generator tubes are advantageously welded to one another in a gastight manner through metal bands or xe2x80x9cfinsxe2x80x9d. The width of the fins influences the heat input into the steam-generator tubes. The fin width is therefore preferably adapted as a function of the position of the respective evaporator or steam-generator tubes in the steam generator to a heating profile which can be predetermined on the gas side. In this case, the heating profile specified may be a typical heating profile determined from empirical values or a rough estimation, such as a stepped heating profile, for example. Due to the suitably selected fin widths, a heat input into all of the evaporator or steam-generator tubes, even during greatly varying heating of various evaporator or steam-generator tubes, can be achieved in such a way that temperature differences at the outlet of the respective evaporator or steam-generator tubes are kept especially small. In this way, premature material fatigue is reliably prevented. As a result, the steam generator has an especially long service life.
In a further advantageous refinement of the invention, the inside diameter of a number of evaporator tubes of the respective first and second combustion chambers is selected as a function of the respective position of the evaporator tubes in the respective first and second combustion chambers. In this way, a number of evaporator tubes of the respective first and second combustion chambers can be adapted to a heating profile which can be predetermined on the gas side. As a result, temperature differences at the outlet of the evaporator tubes of the respective first and second combustion chambers are kept small in an especially reliable manner.
A common inlet collector system is advantageously connected in each case upstream of a number of evaporator tubes, which are connected in parallel and which are assigned to the first or the second combustion chamber, for the flow medium, and a common outlet collector system is advantageously connected in each case downstream of the evaporator tubes. This embodiment of a steam generator permits a reliable pressure balance between the evaporator tubes connected in parallel and thus permits an especially favorable distribution of the flow medium during the flow through the evaporator tubes. In this case, a line system provided with choke fittings may be connected upstream of the respective inlet collector system. As a result, the rate of flow of the flow medium through the inlet collector system and the evaporator tubes connected in parallel can be set in an especially simple manner.
The evaporator tubes of the end wall of the respective first or second combustion chambers are advantageously connected on the flow-medium side upstream of the evaporator tubes of the side walls of the respective first or second combustion chambers. As a result, especially favorable cooling of the end wall of the respective first and second combustion chambers is ensured.
A number of superheater heating surfaces which are disposed approximately perpendicularly to the main flow direction of the heating gas and the tubes of which are connected in parallel for a throughflow of the flow medium, are advantageously disposed in the horizontal gas flue. These superheater heating surfaces, which are disposed in a suspended type of construction and are also designated as bulkhead heating surfaces, are mainly heated in a convective manner and are connected on the flow-medium side downstream of the evaporator tubes of the respective first and second combustion chambers. As a result, especially favorable utilization of the heating-gas heat supplied through the burners is ensured.
The vertical gas flue advantageously has a number of convection heating surfaces which are formed from tubes disposed approximately perpendicularly to the main flow direction of the heating gas. These tubes of a convection heating surface are connected in parallel for a throughflow of the flow medium. These convection heating surfaces are also mainly heated in a convective manner.
The vertical gas flue advantageously has an economizer in order to also ensure especially effective complete utilization of the heat of the heating gas.
The advantages achieved by the invention reside in particular in the fact that, due to the concept of a modular construction of the combustion chamber of the steam generator, the latter requires especially little outlay in terms of design and manufacture. Instead of the respective restructuring of the dimensioning of the combustion chamber, the intention now is only to add or remove one or more combustion chambers when constructing the combustion chamber of the steam generator for a predetermined output range and/or a certain fuel quality. In this case, starting from a certain rating of the steam generator, instead of one combustion chamber to be reconstructed, two or more combustion chambers having a smaller output may be connected in parallel on the gas side upstream of a common horizontal gas flue.
Other features which are considered as characteristic for the invention are set forth in the appended claims.
Although the invention is illustrated and described herein as embodied in a fossil-fuel-fired steam generator, it is nevertheless not intended to be limited to the details shown, since various modifications and structural changes may be made therein without departing from the spirit of the invention and within the scope and range of equivalents of the claims.
The construction and method of operation of the invention, However, together with additional objects and advantages thereof will be best understood from the following description of specific embodiments when read in connection with the accompanying drawings.